1. Field of the Invention
The present invention relates to a carrier for a dry two-component developer which is employed in electrophotography, electrostatic recording and electrostatic printing, and also to a method of producing the carrier.
2. Discussion of Background
Conventionally a cascade development method as disclosed in U.S. Pat. No. 2,618,552 and a magnetic brush development method as disclosed in U.S. Pat. No. 2,874,063 are known as methods for developing latent electrostatic images to visible images by use of toner. In any of these development methods, a dry two-component developer is employed.
Such a dry two-component developer is composed of relatively large carrier particles and fine toner particles which are triboelectrically held on the surface of the relatively large carrier particles by the electric force generated by the friction between the carrier particles and toner particles. When such a dry two-component developer is brought near latent electrostatic images, the toner particles are attracted to the latent electrostatic images, with the bonding force between the carrier particles and the toner particles being overcome by the attracting force of the electric field formed by the latent electrostatic images for bringing the toner particles towards the electrostatic images, so that the toner particles are deposited on the latent electrostatic images, whereby the latent electrostatic images are developed to visible toner images. With the toner particles being replenished to the developer in accordance with the consumption thereof during the development, the developer is repeatedly used.
For the above-mentioned development, it is required that the toner particles be provided with accurate chargeability and charge quantity as to be selectively attracted to a desired image area formed on a photoconductor. Furthermore, it is required that the carrier be capable of always triboelectrically charging the toner particles to the desired polarity with a sufficient charge quantity for the formation of images with high quality over a long period of time.
However, in conventional developers, during the process of making a number of copies, there takes place a so-called "spent phenomenon" that a toner film is formed on the surface of the carrier particles by a collision between the toner particles and the carrier particles, or by a mechanical collision between such developer particles and mechanical portions of a development unit, or by the heat generated by such collision, so that the charging performance of the carrier particles is decreased and the toner particles are scattered while in use, causing the deposition of the toner particles on the background of the images and lowering the copy quality. When this spent phenomenon excessively develops, there occurs the case where the developer must be exchanged with a fresh developer in its entirety.
Frequent exchange of the developer would lead to an increase in the copy making cost, so that it has been proposed that the surface of carrier particles be coated with a resin having low surface energy such as silicone resin or fluorine-containing resin to prevent the occurrence of the spent phenomenon for extending the usable period of the developer without being exchanged, and such carrier particles are used in practice.
Such a resin-coated carrier is produced by dissolving a coating resin in a ketone such as acetone or methyl ethyl ketone, an aromatic hydrocarbon such as toluene or xylene, or an organic solvent such as dioxane or tetrahydrofuran to prepare a coating resin solution, and by coating a carrier core material with the thus prepared coating resin solution, for instance, by an immersing method or a spray coating method.
The organic solvents employed for the above-mentioned coating have relatively low boiling points, so that they have the risks of explosion and having adverse effects on human body if inhaled during the coating process. Furthermore, an apparatus for recovering used solvents is necessary, so that the production of such a resin-coated carrier will be costly.
From the viewpoint of the prevention of environmental pollution, a carrier coated with an aqueous polyurethane resin composition has been proposed as disclosed in Japanese Laid-Open Patent Application 5-127431.
Unquestionably, the use of the above-mentioned aqueous resin composition will reduce the risks of the explosion and adverse effects on human body during the production of the carrier, and will eliminate the necessity for recovering the solvent, resulting in the reduction of the production cost.
However, when the aqueous polyurethane resin composition is employed, its surface energy is so high that the carrier coated with the polyurethane resin is poor in the anti-spent phenomenon performance and therefore not suitable for use with a developer with high durability.